To give a common, conventional measure taken when a failure occurs in an EPS device, assisted steering is stopped and shifted to manual steering. Along with increase in vehicle size, however, sudden shift to the manual steering is said to be dangerous, and there is a growing demand to prevent sudden stop of the assisted steering during driving.
With a view to responding to such a demand, in the system configuration of Patent Document 1, for example, the provision of two systems for inverters that drive a motor can cover a failure of one system. In this system that drives the motor with plural coil sets by use of inverters of plural systems, in case of intersystem short-circuit (short-circuit between phase lines or in the motor, etc.), outputs of the respective systems interfere with one another. This hinders desired control of power supply to the motor.
To overcome the above defect, Patent Document 2 discloses setting different voltage values for a center voltage of a pulse width modulation (PWM) signal output from an inverter of a first system and that output from an inverter of a second system so as to provide a potential difference between them to allow a larger amount of current to flow from the inverter of the first system to the inverter of the second system upon intersystem short-circuit. Consequently, the sum of current detection values in three phases, which is basically “0”, deviates from zero, by which it can be determined that an intersystem short-circuit has occurred.